Sebastián Zúñiga

Beam shaping for laser-based adaptive optics in astronomy

The availability and performance of laser-based adaptive optics (AO) systems are strongly dependent on the power and quality of the laser beam before being projected to the sky. Frequent and time-consuming alignment procedures are usually required in the laser systems with free-space optics to optimize the beam. Despite these procedures, significant distortions of the laser beam have been observed during the first two years of operation of the Gemini South multi-conjugate adaptive optics system (GeMS). A beam shaping concept with two deformable mirrors is investigated in order to provide automated optimization of the laser quality for astronomical AO. This study aims at demonstrating the correction of quasi-static aberrations of the laser, in both amplitude and phase, testing a prototype of this two-deformable mirror concept on GeMS. The paper presents the results of the preparatory study before the experimental phase. An algorithm to control amplitude and phase correction, based on phase retrieval techniques, is presented with a novel unwrapping method. Its performance is assessed via numerical simulations, using aberrations measured at GeMS as reference. The results predict effective amplitude and phase correction of the laser distortions with about 120 actuators per mirror and a separation of 1.4 m between the mirrors. The spot size is estimated to be reduced by up to 15% thanks to the correction. In terms of AO noise level, this has the same benefit as increasing the photon flux by 40%.

Laboratory validation of a laser shaping system before guide star projection

Multiple sodium laser beacons are a crucial development in multi-conjugate adaptive optics systems that offers wide-field diffraction limited adaptive optics correction to the astronomical community. This correction is strongly dependent on the laser beam power and quality, so a beam shaping concept is currently being developed to speed-up calibration and alignment of the laser before every run. A method previously reported, has now been implemented on a laboratory bench using MEMS deformable mirrors. Necessary calibration and characterization of the deformable mirrors are described and the results for experimental amplitude correction are presented.

Closed-loop control for tip-tilt compensation on systems under vibration

Mechanical vibrations affect the performance in modern adaptive optics systems. These structural vibrations induce aberration mainly in tip-tilt modes that reduce the accuracy of the astronomical instrument. Therefore, control actions need to be taken. With this purpose we present a laboratory demonstration of vibration rejection of tip-tilt modes using closed-loop control, inducing vibration on the test bench via an eccentric motor with controllable frequency, in order to simulate the structural vibrations mentioned above. We measure the laser vibration and its tip-tilt aberration using a camera and a Shack Hartmann Wave Front Sensor. The control action is carried out by a Fast Steering Mirror (FSM).

Vibrations in MagAO: frequency-based analysis of on-sky data, resonance sources identification, and future challenges in vibrations mitigation

Frequency-based analysis and comparisons of tip-tilt on-sky data registered with 6.5 Magellan Telescope Adaptive Optics (MagAO) system on April and Oct 2014 was performed. Twelve tests are conducted under different operation conditions in order to observe the influence of system instrumentation (such as fans, pumps and louvers). Vibration peaks can be detected, power spectral densities (PSDs) are presented to reveal their presence. Instrumentation-induced resonances, close-loop gain and future challenges in vibrations mitigation techniques are discussed.

Closed Loop for Tip-Tilt Vibration Mitigation

Mechanical vibration affects the performance in modern Adaptive Optics systems. In this work we present a demonstration of a vibration mitigation control system for tip and tilt modes using a Fast Steering Mirror.

Model Predictive Control for Laser Beam Shaping

Adaptive Optics (AO) is a technique used to mitigate the effect of the atmosphere in the resolution of scientific images. The performance of an adaptive optics system strongly depends on the quality of the laser beam projected to the sky in terms of amplitude and phase. Currently, cumbersome procedures are carried out to optimize the laser beam. This paper presents the performance of a Model Predictive Controller (MPC) to adjust the quality of the laser in a system of two deformable mirrors. The Results shows that the MPC effectively corrects the amplitude and phase of the laser beam using deformable mirrors with about 140 actuators and a separation between mirrors z = 3 [m].

Closed-loop control for laser beam shaping system before guide star projection

The adaptive optics system performance depends on multiple factors, including the quality of the laser beam before being projected to the mesosphere. Cumbersome procedures are required in the laser system to optimize the laser beam in terms of amplitude and phase. However, aberrations of the laser beam are still detected during the operations. The performance of laser projection systems can be improved compensating the effects of aberrations in the laser source or misalignment in the transfer optics before the laser beam propagating through the aperture. Despite the algorithm previously reported predict effective amplitude and phase correction is strongly dependent of an accurate DM characterization and transfer optics alignments. The use of feedback makes the system response better in presence of modeling error and external disturbances. A 2-DM closed loop approach for amplitude and a phase correction is designed. Finally the results of simulations and comparisons are discussed.

Vibrations in MagAO: resonance sources identification and first approaches for modeling and control

The Magellan Telescope Adaptive Optics System (MagAO) is subject to resonance effects induced by elements within the system instrumentation, such as fans and cooling pumps. Normalized PSDs are obtained through frequency-based analysis of closed-loop on-sky data, detecting and measuring vibration effects. Subsequently, a space-state model for the AO loop is obtained, using a standard AO loop scheme with an integrator-based controller and including the vibration effects as disturbances. Finally, a new control alternative is proposed, focusing on residual phase variance minimization through the design and simulation of an optimal LQG control approach.